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Bangalore: Two leading
geologists have warned that a magnitude 6-plus earthquake cannot
be ruled out in Jaitapur - the proposed site of India's largest
9,900 MW nuclear power plant on the west coast that has seen
protests against it for safety reasons - and that it could occur
within the lifetime of the power plant.
"Since Jaitapur lies in the same compressional stress regime that
has been responsible for generating both the magnitude 6.3 Latur
and magnitude 6.4 Koyna earthquakes in the past five decades, it
can be argued that a similar sized earthquake could possibly occur
directly beneath the power plant," they say in a report in the
latest issue of Current Science published by the Indian Academy of
Sciences in Bangalore.
"The probability of this earthquake occurring is low but it is
nevertheless possible, and is an important consideration in the
analysis of power plant safety," say its authors -- Roger Bilham
in the department of geological sciences of the University of
Colorado, US, and Vinod Gaur in the CSIR Centre for Mathematical
Modelling and Computer Simulation in Bangalore.
According to Bilham and Gaur, the Indian Plate is unique among the
world's continental plates in that it is flexed by its collision
with the Tibetan Plateau resulting in "belts" of buckling parallel
to the Himalaya that extend southward, deep into the plate
interior.
This "flexural depression", they say, results in high
compressional stresses that are believed to be responsible for the
thrust faulting that produced the Latur earthquake, and presumably
for the faulting in the Koyna region. "The Jaitapur region lies in
this same compressional downwarp," the researchers warn.
"The occurrence of earthquakes of up to magnitude 6.5 on faults
near Koyna and Latur at approximately the same latitudes of
Jaitapur is of considerable concern, since the stress regime near
Jaitapur cannot differ substantially from these two areas when
viewed from a 'flexural' perspective," the scientists note.
Moreover, the occurrence of the nearby Koyna earthquake has
presumably loaded the Jaitapur region closer to failure as a
result of a process called "Coulomb stress transfer", they add.
The Jaitapur site is 110 km from the Koyna earthquake of 1967,
which was induced by the impounding of the Koyna reservoir. The
authors say the Koyna earthquake signifies a region that was
highly stressed prior to reservoir impoundment.
"As the stresses in the region are likely to be similar over
hundreds of kilometres, the Jaitapur region must be considered to
be similarly stressed," they say.
Jaitapur has no record of local seismicity in the past century.
However, moderate events such as the Koyna earthquake at distances
less than 30 km and larger ones at distances greater than 100 km
can also produce significant shaking, they say.
This level of shaking can be easily accommodated by most nuclear
power plants, but considering that Jaitapur lies in a tectonic
setting similar to Latur and Koyna, an earthquake of magnitude 6.5
is "not unlikely" there, they say. "It could occur within the
lifetime of the nuclear power plant."
The Nuclear Power Corporation that plans to house six French
reactors in Jaitapur has said the reactors will not face a seismic
risk as the site is in seismic zone three and not four. But Bilham
and Gaur note that the historical seismic record near Jaitapur
extends reliably back for only 200 years and estimates of risk
assessed from a short dataset of only the past few centuries, "may
not represent the true risk to the plant", they say.
Bilham and Gaur say the low strain rate and the rare incidence of
recent or historical earthquakes in the region means "there
presumably exist numerous faults that represent seismic hazards we
know nothing about". Many of India's faults do not reach the
surface, and their examination using traditional palaeo-seismic
methods is not feasible, they say.
"A knowledge of the distribution of surface and subsurface faults
near Jaitapur is, therefore, an important factor in characterising
local seismic hazards considering that the Latur earthquake
occurred on an unmapped surface fault," the report says.
The geologists caution that the apparent "seismic quietness" of
Jaitapur does not mean that a severe earthquake cannot occur
there. "If stress in the region is sufficiently mature to have
brought an existing subsurface fault close to failure, an
earthquake may be imminent," they said. "It is our opinion that
insufficient data are available to exclude this possibility."
"While this may be considered of low probability, it is
nevertheless possible, and as the recent earthquake in Japan has
demonstrated, it is relevant to plan for all possible futures in
the design of nuclear power plants," the geologists conclude.
(K.S. Jayaraman can be contacted at killugudi@hotmail.com)
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